1. Field of the Invention
This invention relates to a method for improving the properties of polymers by subjecting them to microwave energy. Specifically, this invention relates to a method for improving the APHA color of a polymer, particularly polytetrahydrofuran by subjecting the polymer to microwave energy.
2. Background of the Invention
The use of microwave energy to treat polymers and their precursors for a variety of purposes is known. U.S. Pat. No. 4,356,071 to Fenton, for example, discloses the use of microwave heating to treat formaldehyde or trioxane prior to polymerization in order to obtain a more stable polymer.
U.S. Pat. No. 4,253,898 to Rinker et al. discloses the use of microwave energy to melt an adhesive composition interposed between and in contact with plastic components so as to cause bonding of the components.
U.S. Pat. No. 4,352,854 to Siedenstrang et al. discloses the use of microwaves to create a foamed rubber with a sponge like interior surrounded by a polymer skin.
U.S. Pat. No. 4,550,125 to Lee et al. discloses the use of microwaves to cure polyorganosiloxanes.
Polymers of tetrahydrofuran ("THF") are used in thermoplastics, castable urethanes, elastomers and spandex fiber. Many downstream applications of THF polymer require a low APHA color.
APHA color is an important characteristic of polymers which reflects the optical clarity of the polymer with respect to its yellowness. APHA color refers to a platinum-cobalt color standard as determined by ASTM 1045-58 which involves visual comparison of a liquid sample of the polymer with solutions having standard concentrations of potassium chloroplatinate and cobaltous chloride. Reduction of APHA color, i.e. reduction in yellowness, is desired because it affects the quality of the final product. Downstream users of polytetrahydrofuran generally mix it with additives, colorants, and other polymers which may be adversely affected by a high level of APHA color, hence the desirability of minimizing APHA color. Up to now, APHA color could be controlled only in the processing stage of polymer manufacture. We know of no method, other than that of the present invention, of treating a polymer after its manufacture to reduce APHA color. It has now been found that treating polymer with microwave radiation significantly improves the APHA color of the polymer.